ai: wire arcadia-search backend (search_kb + read_chunk + reindex button)

Adds the agent-facing surface for the new Tantivy lexical search service
(arcadia-search). Sits alongside the existing search_docs (browser
MiniSearch) — agent picks based on tool description.

- admin-tools.ts: new search_kb(query, corpus, limit?, tags?) and
  read_chunk(chunk_id, corpus) tools. KB_BASE_URL honors
  window.__ARCADIA_SEARCH_URL runtime override + VITE_ARCADIA_SEARCH_URL
  build env, defaults to localhost:7800. Token resolved per-call from
  sessionStorage.arcadia_access_token (matching lib-arcadia-client's
  storage convention) with "dev" fallback for unauthenticated dev.
- assistant.tsx: system-prompt section telling the agent when to pick
  search_docs (browser, bundled) vs search_kb (server, dynamic +
  expandable via read_chunk).
- ai.tsx: reindexKB() helper + "reindex kb (docs)" button on the empty
  state, next to the existing block-preview button. Toasts on
  start/success/failure. Wired with data-action="kb-reindex-docs" so
  the agent can also trigger via the command bus.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/routes/assistant.tsx

@@ -113,6 +113,7 @@ function buildAdminPreface(activeAgent: Agent | undefined, uiControl: boolean):
   const ctx = formatAdminContextForPrompt()
   const parts = [
     "You are the operator's assistant inside Arcadia Admin. Be precise and direct. You have native function tools attached to this conversation — call them whenever the user asks about live platform state (counts, statuses, listings, lookups). Never invent tenant slugs, user counts, or statuses; if you need data, call a tool.",
+    "Two retrieval surfaces exist for documentation/knowledge: `search_docs` (browser-side, BM25 over the bundled arcadia docs — fast, always available, small corpus) and `search_kb` (server-side, BM25 over arcadia-search — same docs as `corpus=docs` for parity, plus larger and additional corpora as the operator adds them). For questions about the bundled arcadia docs either is fine; prefer `search_kb` when you want richer hits or when the user is asking about content that wouldn't be in the bundled docs (uploaded files, tenant-specific knowledge). When `search_kb` returns a chunk_id you want to expand, call `read_chunk(chunk_id, corpus)`.",
     RICH_OUTPUT_PREFACE,
     ARCADIA_KNOWLEDGE,
     persona,